Junior Software Engineer Interview Questions

The Junior Software Engineer Interview Process

Coding, Debugging & Working in Codebases

These questions test your ability to work within an established codebase, understand unfamiliar code, debug real problems, and collaborate through version control.

• You're assigned to fix a bug in a large codebase you've never seen before. Walk me through your debugging approach.

  What interviewers look for: A strong answer sounds like: 'I'd start by reproducing the error locally using the same steps, then check the stack trace to find where the failure occurs. Next, I'd read the existing tests to understand the expected behavior, trace the code path with a debugger, add logging if needed, and verify the fix doesn't break other tests before submitting a PR.' A weak answer sounds like: 'I'd just try different things until it works' or 'I'd ask my mentor what to do.'

  Sample Answer Guidance: I'd first reproduce the error using the same steps that triggered it, then examine the stack trace to pinpoint where the failure occurs. I'd read the failing test and any related tests to understand expected behaviour, trace through the code path with a debugger or logging, and then verify my fix doesn't break other tests before raising a PR.
• A feature you shipped last week is causing slow API response times in production. How would you investigate and fix it?

  Interviewer Insight: Debugging questions at junior level test your PROCESS, not your speed. Interviewers want to see that you have a systematic approach—reproduce, isolate, fix, verify—not that you guess randomly. They're evaluating whether you'd panic under pressure or methodically solve problems that affect real users.

  Sample Answer Guidance: I'd check the server logs and application metrics to see when the slowdown started and correlate it with my deployment. Then I'd profile the code using tools like New Relic or Chrome DevTools to identify bottlenecks—likely an N+1 query or missing database index. I'd test locally to reproduce, implement a fix (e.g., eager loading or adding an index), and monitor metrics before deciding whether to roll back.
• You're asked to refactor a 500-line function into smaller, more testable pieces. Where do you start?

  Sample Answer Guidance: I'd first ensure the function has comprehensive tests so I can verify behaviour doesn't change during refactoring. Then I'd identify logical boundaries—perhaps loops, conditionals, or business logic blocks—and extract them as smaller helper functions incrementally, running tests after each change. This keeps the code working whilst making it more testable.
• Describe your workflow when creating a feature branch, making commits, and submitting a pull request. What mistakes have you made with Git?
• You're reviewing a PR from a teammate and you spot a potential issue, but you're not 100% sure. How do you raise it?

  Interviewer Insight: Code review questions reveal your coachability. Interviewers are testing whether you can receive feedback gracefully and whether you can articulate why code should change, not just what should change. A junior who handles reviews well becomes a peer who strengthens the entire team.
• Walk me through how you'd approach reading and understanding unfamiliar code in a new codebase.

  Sample Answer Guidance: I'd start by finding the entry point—the main function or API endpoint—and trace the data flow through the code. I'd use the IDE's search to find function definitions, read related test files to understand expected behaviour, and follow imports to understand how modules connect. If I'm stuck, I'd ask a teammate rather than spend hours guessing.
• Your code works locally but fails in the staging environment. What are the first things you'd check?
• Explain how you'd use version control to isolate your changes and ensure you're not accidentally modifying code outside your scope.

Code Review, Testing & Growth

These questions assess your ability to write testable code, give and receive feedback in code reviews, and develop skills needed to progress toward mid-level engineering.

• Describe your approach to writing a unit test for a function that depends on an external API. What challenges did you face?

  Sample Answer Guidance: I'd mock the API response using a library like jest.mock() or unittest.mock, so the test doesn't depend on the actual service being available. I'd test both success and failure paths—what happens when the API returns an error or times out. The challenge is ensuring the mock accurately mirrors the real API's behaviour, so I'd version the mock alongside API changes and update tests when the contract changes.
• You're reviewing a PR that uses a complex algorithm you don't fully understand. How do you approach the review?

  Sample Answer Guidance: I'd first check if the algorithm has comprehensive tests covering edge cases—if yes, that gives me confidence. Then I'd ask the author to explain the approach and add inline comments explaining the key logic. I'd suggest we pair on a walk-through if needed, or escalate to a senior engineer if the change is risky. Being honest about gaps in understanding is better than approving code I don't trust.
• Tell me about a time you suggested changes to someone else's code. How did you phrase your feedback, and what was the outcome?
• You've written code that passes tests but has poor performance. How would you identify this, and what would you do?

  Sample Answer Guidance: I'd use profiling tools like Python's cProfile or Chrome DevTools to identify bottlenecks—which functions consume the most time. I'd check algorithmic complexity (Big O) to understand if I'm doing something O(n²) that should be O(n log n). Then I'd run load tests to reproduce the issue at scale, fix it, and monitor production metrics to ensure the optimisation works in real conditions.
• Describe your experience with a testing framework or tool (e.g., Jest, pytest, RSpec). What did you like or dislike about it?

  Sample Answer Guidance: I've used Jest for JavaScript—I like its snapshot testing and built-in mocking, but snapshots can obscure what you're actually testing if not used carefully. When choosing a framework, I'd consider the language ecosystem, team familiarity, and whether it supports the testing patterns we need (mocking, async, fixtures). Convention and documentation matter as much as features.
• What skills or areas of software engineering do you want to improve over the next 12 months?

  Interviewer Insight: Growth questions are the most important at junior level. Interviewers hire juniors who will become mid-level engineers in 18 months, not juniors who will stay junior. They're evaluating whether you're introspective, ambitious, and coachable enough to take feedback and run with it.
• Tell me about a code review comment that helped you grow as an engineer. What did you learn?

How Interviewers Evaluate Junior Software Engineers